This invention relates to a thread guage, particularly a thread guage for indirectly and simultaneously measuring the pitch diameter and taper of threaded pipe ends.
In the production of oil and gas, pipe segments connected in a continuous "string" of tubing or pipe are utilized for transporting well fluids from the producing formation to the surface and for transporting well fluids from location to location on the surface. The connection between the pipe segments is very important to the proper performance of the pipe string. Threads are provided at each end of each pipe segment for joining the pipe segments into a continuous string of pipe. Different sizes of pipe are utilized in the completion of an oil well and therefore several different thread configurations are applied to the pipe segments. Primarily, all thread configurations satisfy at least two common criteria; they must resist leakage and tensile failures. Proper connection between pipe segments is, therefore, of major importance to proper performance of the pipe string. To accomplish this in a variety of circumstances various threads have been specifically designed for use in the oil industry. The threads are machined on the pipe end. Proper thread cutting is verified by laboratory and prototype testing prior to marketing. Also, field inspection of threads is routinely made to assure that the connection between pipe segments can be dimensionally accurate.
In order to insure proper thread connections, the size and configuration of certain non-proprietary threads used on the end of pipe segments have been standardized by the oil industry. In addition, proper connection procedures used for joining these pipe segments have been standardized for connecting specific pipe configurations. The standards specifying the size configuration and orientation of threads on the ends of pipe segments used in the oil and gas industry are set forth by the American Petroleum Institute (API) in its Specification For Threading, Guaging, And Thread Inspection Of Casing, Tubing, And Line Pipe Threads. This publication sets forth the standard dimensions characteristic of the various API thread configurations. In particular, the pitch diameter and taper characteristic of various size threaded pipe is specified.
The thread design is verified at the pipe mill and guaged for conformance with API standards. However, field inspection of the pipe threads is routinely done to determine thread damage which may have occurred during shipment and also to identify any off-specification threads not found at the mill. It is important to locate damaged or off-specification pipe. However, rejecting "good" pipe results in a financial burden on the mill and/or the operator. Thus, inspection must be performed by qualified personnel according to prescribed standards.
The published industry standards for determining conformity of threads provide certain standard guaging procedures for measuring the critical dimensions and configurations of threads. The published procedure also includes a discussion of the proper use of various guaging devices. Generally, two types of guages are advocated for thread inspection; dial indicating guages and fixed guages. The dial indicator guages are provided with a dial indicator, which when placed on the thread must read within a certain range if the thread is within specification. The fixed guage is a rigid guage which is screwed onto the thread. The thread is properly machined if the other thread elements are within specification and the fixed guage stand-off is within specification. Compliance with standard guaging procedures and use of recommended guaging devices is generally accepted in the industry for field inspection of pipe threads and accurately identifying at least acceptable threaded pipe within an acceptable range of the standardized specifications. However, prior art guaging devices may give unreliable readings. For example, API ring and plug guages "assume" a perfectly round thread form. If the pipe segment is out-of-round, the thread form will be out-of-round resulting in an erroneous reading from the ring or plug guage.
The types of guages typical of the prior art used in field inspection of threads have now always provided an accurate indication of the acceptability of a thread being measured for use in a pipe string. For example, use of the prior art devices, according to the standardized procedure, results only in a determination of whether a particular thread is acceptable within the specified standards. The prior art devices may reject the thread forms, but they do not quantify the error. The prior art devices are used to either accept or reject a specific thread as being within the prescribed standard. They do not determine the compatibility of threads, particularly when the threads are not within standard specifications.
The thread guages described in U.S. Pat. No. 2,840,475 to McGaffey and U.S. Pat. No. 4,189,843 to Baldwin are typical of the prior art devices which are capable of measuring thread dimensions of threads formed on the ends of pipe. The devices described by McGaffey and Baldwin are adjustable for identification of a predetermined or sought pitch diameter and taper. The devices are preset and placed on a threaded pipe end. If the thread is within standard specifications, the thread guage will contact the thread at a specified point. Proper use of the thread guages requires accurate presetting of the devices and determination of the contact point with the thread being measured. Thus, repetition of results is difficult to obtain, particularly when different personnel use the devices.